chore: address concurrency warning on onPreferenceChange

In the SDKs released with Xcode 16.2, Apple marked this closure
`@Sendable`, but *not* `@MainActor`. It's not clear if this is
intentional or if it may be revised later. The official
recommendation appears to be capturing the `Binding` and writing
to it (`Binding` is asserted to be `Sendable`).
https://stackoverflow.com/questions/79209071/xcode-16-2-beta-2-renders-onpreferencechange-unusable

Author: cweider

Sources/MarkdownUI/Utility/ResizeToFit.swift

@@ -34,8 +34,9 @@ private struct ResizeToFit1<Content>: View where Content: View {
         .preference(key: SizePreference.self, value: size)
     }
     .frame(width: size?.width, height: size?.height)
-    .onPreferenceChange(SizePreference.self) { size in
-      self.size = size
+    .onPreferenceChange(SizePreference.self) { [$size] size in
+      // Roundabout capture makes closure sendable (see #389)
+      $size.wrappedValue = size
     }
   }
 

Sources/MarkdownUI/Views/Blocks/BlockSequence.swift

@@ -26,8 +26,9 @@ where
     VStack(alignment: self.textAlignment.alignment.horizontal, spacing: 0) {
       ForEach(self.data, id: \.self) { element in
         self.content(element.index, element.value)
-          .onPreferenceChange(BlockMarginsPreference.self) { value in
-            self.blockMargins[element.hashValue] = value
+          .onPreferenceChange(BlockMarginsPreference.self) { [marginBinding = $blockMargins[element.hashValue]] value in
+            // Roundabout capture makes closure sendable (see #389)
+            marginBinding.wrappedValue = value
           }
           .padding(.top, self.topPaddingLength(for: element))
       }

Sources/MarkdownUI/Views/Blocks/ColumnWidthPreference.swift

@@ -9,7 +9,8 @@ extension View {
     )
   }
 
-  func onColumnWidthChange(perform action: @escaping ([Int: CGFloat]) -> Void) -> some View {
+  func onColumnWidthChange(perform action: @escaping @Sendable ([Int: CGFloat]) -> Void) -> some View {
+    // Roundabout capture makes closure sendable (see #389)
     self.onPreferenceChange(ColumnWidthPreference.self, perform: action)
   }
 }

Sources/MarkdownUI/Views/Blocks/NumberedListView.swift

@@ -41,8 +41,8 @@ struct NumberedListView: View {
     )
     .environment(\.listLevel, self.listLevel + 1)
     .environment(\.tightSpacingEnabled, self.isTight)
-    .onColumnWidthChange { columnWidths in
-      self.markerWidth = columnWidths[0]
+    .onColumnWidthChange { [$markerWidth] columnWidths in
+      $markerWidth.wrappedValue = columnWidths[0]
     }
   }
 }